iOS support

[jbaicoianu]

Now that iOS is finally showing signs of modernizing their 3d support (WebGL2, WebXR), we should do some testing and make sure the engine still works. iOS has been unsupported for a while with some people reporting that they get a black screen, likely due to some API we're using that Safari doesn't support.

[jbaicoianu]

Thanks to @foxlet we were able to narrow down that this is likely due to the use of float textures in the postprocessing render codepath. Disabling postprocessing entirely fixes the issue - now debugging to see what we can do to handle this gracefully without disabling postprocessing entirely.

[jbaicoianu]

The render now initializes properly on iOS, but Safari is still much more sensitive to memory usage than any other mobile browser, and regularly crashes on all but the simplest of scenes. Need to do a pass to see what we can do to optimize memory usage.